The subject of the invention is a test device for measuring the electromagnetic radiation, immunity and characteristics of an object to be tested, comprising a conducting structure enclosing a test volume in which said object may be placed, said structure being provided with a closable aperture, said test device also including a system of internal conductors called septa, these being at least two in number, each septum being composed of at least one elementary conductor, each septum being connected to at least one and to at most two connectors passing through the structure, each connector being connected to at least one septum, each septum defining, in said test volume, by its arrangement in the structure, an electromagnetic coupling with said object, said electromagnetic coupling being specified by an electric coupling vector and a magnetic coupling vector defining an electromagnetic coupling plane.
Such devices are known and used at the present time, these being called transverse electromagnetic cells (TEM cells) which are distinguished principally by the geometry of their metal structure, by the number, shape and construction of the septa, by the arrangement of said septa in the structure and by the number of connectors.
The publication IEEE Transactions on Electromagnetic Compatibility Vol. 16, No. 4, November 1974: xe2x80x9cGeneration of Standard EM Fields using TEM Transmission Cellsxe2x80x9d (M. Crawford) describes the conventional TEM cell and its use for creating electromagnetic fields for the purpose of measuring the electromagnetic immunity of objects to be tested. The metal structure, of polyhedral shape, of this cell comprises a central section of parallepipedal shape between two pyramidal transition sections. Placed at each end of the transition sections is a connector that connects the two ends of a septum of hexagonal shape located at mid-height inside the structure. This cell is limited only in terms of high frequencies by the appearance of resonant modes that depend on the dimensions of the cavity formed by the metal structure, above which frequencies the transverse electromagnetic coupling mode is no longer verified.
The National Bureau of Standards Report NBS No. TN-1059 (PB83-165274) of October 1982: xe2x80x9cA Method to Quantify the Radiation Characteristics of an Unknown Interference Sourcexe2x80x9d (M. T. Ma and G. H. Koepke) describes a method of measuring electromagnetic radiation using a conventional TEM cell with one septum and two connectors, in which the object on test is placed in succession in several positions.
U.S. Pat. No. 4,837,581 describes a TEM cell of pyramidal shape whose septum of triangular shape is connected, on the one hand, to a connector placed at the end of the structure formed by the apex of the pyramidal shape and, on the other hand, to an array of loads which is connected to the other end of the structure formed by the base of the pyramidal shape. This cell, called a GTEM cell, has the feature of also possessing a system of elements that absorb the electromagnetic waves on the internal wall connected to the array of loads, making it possible to use it at frequencies well above those of the conventional TEM cell.
U.S. Pat. Nos. 5,754,054; 5,404,098; 5,825,331 and 5,430,456 describe methods of measuring electromagnetic radiation which are distinguished from the measurement method described in NBS Report No. TN-1059 by the number of positions and the orientations of the object to be tested and by the use of a GTEM cell.
U.S. Pat. No. 5,589,773 describes a radiation measurement method and a positioning system suitable for a GTEM cell, said positioning system making it possible to facilitate and automate the changes of orientation of the equipment under test.
Patent JP 05312866A describes a conventional TEM cell comprising a system for inserting the object to be tested, while Patent JP 05005763A describes a conventional TEM cell comprising, in the internal volume of the structure, elements intended for stirring the electromagnetic radiation.
U.S. Pat. No. 5,585,808; DE 3 925 247 A1; JP 02203281A; JP 04353774A; JP 05264620A and JP 06242161A describe TEM cells having a septum which are distinguished by the geometrical shape of their structure and by the geometrical shape and the arrangement of the septum inside the structure. Patent JP 10267975A describes a TEM cell which is distinguished from the cell described in patent JP 06242161A in that the internal walls of the structure are covered with a material that absorbs the electromagnetic energy.
U.S. Pat. No. 5,910,729 describes a TEM cell having two septa, each of them being connected at its two ends to a connector. The symmetrical arrangement of the two septa is designed to improve the electromagnetic coupling performance. U.S. Pat. No. 5,942,903 describes a TEM cell which is essentially distinguished from the cell described by U.S. Pat. No. 5,910,729 by the shape of the structure, by the particular construction of the septa and by their connection at one end to an array of loads. U.S. Pat. No. 5,861,753 describes a TEM cell which is distinguished from that described by U.S. Pat. No. 5,942,903 by the shape of the structure and in that it has three septa and two connectors, two of the three septa being arranged symmetrically in the structure and connected to only one of the connectors via a coupler.
U.S. Pat. No. 5,793,215 describes a TEM cell which is distinguished from the TEM cell described by U.S. Pat. No. 5,910,729 in that it has three septa of cylindrical shape which are connected to the two ends of the structure via six bushing connectors. Patent JP 11174102A describes a TEM cell which is essentially distinguished from the TEM cell described by U.S. Pat. No. 5,793,215 in that the three septa are of flat shape.
Patent JP 10185981A describes a TEM cell of mainly cylindrical shape, comprising a septum that rotates about the longitudinal axis of the structure, said septum being connected to a connector at each end of the structure. Furthermore, the TEM cell includes a rotary support for rotating the object on test about the vertical axis. Patent DE 196 01 348 C1 describes a TEM cell having a single two-position rotary septum.
U.S. Pat. Nos. 5,327,091 and 5,530,412 describe two methods of stirring the modes of a resonant cavity formed by a closed metal structure not provided with a septum, called a mode-stirring reverberation chamber inside which are placed in particular the object on test and an antenna transmitting electromagnetic energy.
In the abovementioned U.S. Pat. Nos. 5,910,729; 5,942,903; 5,861,753; 5,793,215 and KR 97-64814 comprising at least two septa, the electromagnetic coupling planes of said septa in the test volume are coincident, that is to say the electric and magnetic coupling vectors associated with each septum are oriented in a plane perpendicular to the longitudinal axis of the TEM cells. This is also the case with the TEM cells having a rotary septum that are described in patents KR 96-57363 and DE 196 01 348 C1, whatever the arrangement of the septum in the cell. In all cases, the electromagnetic characterization of the objects under test is therefore carried out in a two-dimensional reference frame of the same electromagnetic coupling plane. The TEM cells described in those patents have mainly been developed for measuring the electromagnetic characteristics of objects in several electromagnetic coupling polarizations within the same plane, in particular the vertical and horizontal polarizations, so as to reproduce the measurements made on an open site or in an anechoic chamber in which the object to be tested is placed at a certain distance from an antenna polarized vertically and then horizontally. The three-dimensional electromagnetic characterization of the object on test can then be obtained only by changing the orientation of the object on test in the test volume, as indicated in the NBS report No. TN-1059 and the U.S. Pat. Nos. 5,754,054; 5,404,098; 5,825,331; 5,430,456 and 5,589,773.
In the mode-stirring reverberation chambers described in the abovementioned U.S. Pat. Nos. 5,327,091 and 5,530,412, the electromagnetic coupling planes in the test volume are produced by a multitude of electromagnetic waves emanating from the transmitting antenna and reflected by the conducting walls of the metal structure. At resonant frequencies of the cavity of the structure, the superposition of all of these coupling planes in the test volume gives the object on test a statistically isotropic character over a mode-stirring cycle, whatever the orientation of said object to be tested. This test device and its operation are nevertheless dependent on the geometrical dimensions of the structure, which define the first resonant frequency of the cavity and on the minimum frequency for which the isotropic character of the object on test is obtained over a stirring cycle.
In the same way as the mode-stirring reverberation chambers, and unlike the known TEM cells, the test device forming the subject matter of the invention makes it possible to avoid manipulating the equipment under test by the particular arrangement of the septa in the structure and in addition allows simultaneous measurements to be taken. This constitutes an advantage of the test devices forming the subject matter of the invention. To achieve this objective, the test device forming the subject matter of the invention has at least two septa, at least one of these septa is placed in such a way that its electromagnetic coupling plane is distinct and not parallel to the electromagnetic coupling plane of at least one other septum.
According to a first variant of the invention, the test device has three septa placed in the structure in such a way that their electromagnetic coupling planes taken in pairs are distinct and nonparallel. Furthermore, according to this first variant, the electromagnetic coupling planes taken in pairs may be orthogonal.
According to a second variant of the invention, the test device has two groups of three septa, the septa of each group being placed in the structure in such a way that their electromagnetic coupling planes taken in pairs are distinct and nonparallel. Furthermore, according to this second variant the electromagnetic coupling planes of each of these groups taken in pairs may be orthogonal. In the test devices forming the subject matter of the invention, the structure may be polyhedral in shape and more particularly parallepipedal in shape or else cubic in shape.
In general, the dimensions of the polyhedral shape of the test device forming the subject matter of the invention may be chosen, on the one hand, in order to fix the first resonant frequency of the cavity of the structure and, on the other hand, in order to increase the number of high-frequency resonant modes of the cavity. Thus, the test device forming the subject matter of the invention could benefit from the complementary properties of TEM cells and of mode-stirring reverberation chambers. In particular, when the structure is parallelepipedal in shape, the ratios of the dimensions of this shape, taken in pairs, may be irrational numbers.
Furthermore, all or part of the internal surface of the structure of a test device forming the subject matter of the invention may be covered with a material that absorbs electromagnetic energy and the internal volume of this structure may include elements intended for stirring the electromagnetic radiation.